Conductive resin materials and plastic materials are currently used for electrical connection between minute sites of electronic devices, e.g., between ITO electrodes and driving LSIs, between LSI chips and circuit boards, and between micro-pattern electrode terminals in liquid crystal display (LCD) panels. In particular, anisotropic conductive films are used to ensure electrical contact between electrodes and reliable connection. As pitch intervals have recently become narrower in conductive films, conductivity, adhesiveness, dispersability and content of conductive particles capable of imparting anisotropic conductivity to the conductive films have gained importance.
For example, resin/metal complex particles useful as conductive particles are prepared by forming a thin metal layer on Ni particles, Ni/Au complex particles or plastic particles as base particles, depending on the particular application of anisotropic conductive films.
Electroless plating has been employed to prepare conductive resin/metal complex particles comprising plastic particles. Generally, conductive resin/metal complex particles are prepared by pretreatments, e.g., defatting, etching, sensitizing, catalyzing, treating with a reducing agent, etc., of base particles or powder, followed by electroless plating (Japanese Patent No. 2507381; Japanese Patent Publication No. 1994-096771; and Japanese Patent Laid-open Nos. 1990-024358, 2000-243132, 2003-064500, and 2003-068143). At this time, the electrical/physiochemical properties of the final particles vary according to the kind and number of metals to be introduced. Ni/Au double complex metal layers are commonly applied to anisotropic conductive films (Japanese Patent Laid-open Nos. 1999-329060 and 2000-243132).
The reason why Ni/Au consecutive metal layers are commonly employed among metal-plated particles using plastic base particles is that Ni can easily be formed into form a thin metal layer by electroless plating, Au can be plated on the surface of the plated Ni by substitution plating, and Au shows stable electrical connection properties at connection sites of semiconductors and other mounting devices due to its superior conductivity.
Japanese Patent Laid-open No. 2000-243132 discloses the formation of a Ni/Au complex plating layer by forming a substantially indiscrete Ni layer on plastic base particles by electroless plating, and forming an Au layer on the Ni layer by substitution plating. The expression “substantially indiscrete Ni layer” as used herein refers to a plating layer having a thickness of 5 nm or more formed by deposition of fine Ni particles during plating when observed under a scanning electron microscope (SEM) at a magnification of 5,000× to 10,000×. The formation of the substantially indiscrete Ni layer is indispensable for the introduction of an Au layer. Taking the plating adhesion to the base particles into consideration, in actuality, a Ni layer having a thickness of from about 50 nm to 70 nm has commonly been introduced.
However, when conductive particles comprising a Ni plating layer within the thickness range defined above are interposed between electrodes and compressively deformed by 10% or more, peeling takes place between the plastic particles and the Ni layer. When the compressive deformation is continued above 10% or more, the peeling phenomenon leads to the rupture of the Ni layer, resulting in poor electrical connection of an anisotropic conductive film. The causes of the peeling phenomenon are that the Ni layer has a relatively high hardness and a relatively low elastic modulus, compared to the plastic particles. Thus, there is a need for a Ni—Au complex conductive layer having superior adhesion to plastic particles and superior electrical properties while maintaining the thickness of a Ni layer at a minimum level.